1. Field of the Invention
The present invention relates to a constant vacuum type carburetor having a vacuum actuated valve driven up and down within an air induction passage by a vacuum pressure generated in the air induction passage on an upstream side of a throttle valve.
2. Description of the Related Art
The conventional constant vacuum type carburetor has been disclosed in a commonly owned Japanese Examined Patent Publication No. Heisei 1(1989)-35173. In the disclosed constant vacuum type carburetor, an air induction passage is formed through a carburetor main body. Also, a guide cylinder of a vacuum actuated valve is extended upwardly from the air induction passage with an upward opening.
Within the guide cylinder of the vacuum actuated valve, a vacuum actuated valve is arranged for up and down movement. On the upper end of the vacuum actuated valve, a diaphragm is arranged the inner periphery thereof is clamped with a diaphragm plate. Thus, a pressure receiving chamber is defined between an upper surface of the diaphragm and a top cover covering the upper side of the diaphragm. Also, an atmospheric pressure chamber is defined by a lower surface of the diaphragm and a recessed portion of the carburetor main body.
On the other hand, within the vacuum actuated valve, a jet needle is arranged to project toward the lower end thereof. Movement of the jet needle in the longitudinal direction is restricted by a jet needle holder which is formed by a thread member or so forth, arranged corresponding to the upper end of the jet needle.
A vacuum actuated valve return spring is arranged within the pressure receiving chamber in compressed position. The lower end of the vacuum actuated valve return spring is directly engaged with the inside of the vacuum actuated valve. On the other hand, the upper end of the vacuum actuated valve return spring is engaged with the top cover.
In the conventional constant vacuum type carburetor, there has been proposed the vacuum actuated valve which is provided with a thin plate shaped cross-section (for example, see Japanese Examined Utility Model Publication No. Showa 43(1968)-23922). This measure has been taken to improve air induction efficiency for an intake air flowing through the air induction passage, or for down-sizing of the carburetor main body. When the thin plate form vacuum actuated valve is employed, a problem is encountered in directly engaging the vacuum actuated valve return spring to the inside of the vacuum actuated valve. This is because a diameter of the vacuum actuated valve return spring becomes small requiring a large spring constant to assure the fine and smooth lifting characteristics of the vacuum actuated valve in response to increasing of vacuum pressure in the pressure receiving chamber. Consequently, a long period is necessary to conduct an adaptation test of the carburetor. Also, installation ease of the vacuum actuated valve return spring into the vacuum actuated valve is degraded. Furthermore, a difficulty is encountered in arranging the jet needle holder within the vacuum actuated valve.
On the other hand, as a material of the vacuum actuated valve, a synthetic resin material has been used for light weight and low production cost. Particularly, in order to maintain dimensional precision and to obtain sufficient mechanical strength, a glass fiber reinforced synthetic resin is used. Also, a diaphragm plate to be fixedly arranged in the vacuum actuated valve by thermal welding is formed of glass fiber reinforced synthetic resin. The reason for this is that, upon thermal welding of the vacuum actuated valve and the diaphragm plate, both members have to be connected by thermal welding.
Similarly to the above, the top cover is also formed of glass fiber reinforced synthetic resin for lightweight and low production cost.
In the constant vacuum type carburetor constructed as set forth above, a fully open position of the vacuum actuated valve is determined by abutting the diaphragm plate of the vacuum actuated valve to the top cover. Therefore, in a long period use, both components formed of glass fiber reinforced synthetic resin can wear.